So it seemed Exchange couldnt accept the message from Ironport because of "Insufficient system resources" (finally after hours the message had been delivered).

Checking the Exchange server memory showed no problem, but the disk where Exchange software is installed (D: in my environment) had only 2.3 GB left.
I have just migrated all the mailboxes in some days and the fastly growing logs (basically IIS and message tracking) were causing my D: drive to be almost full. Newbie failure 🙁

So, steps I have performed to solve this issue:
– To quickly restore normal flow of the email, temporary moved the logs to C: drive that had enough space to generate no problems with that movement
– Assigned a new SAN LUN to each of the Exchange servers so I can configure a new drive to store the logs. If for any reason Im not aware of the disk getting full, this wouldnt cause any problem to the mail flow service
– Once configured the new drive I could configure the Exchange logs paths pointing to the new drive and move the logs from C: drive to the new location

Additionally, I have to… monitor the disk space!

So, It is interesting to change Exchange logs paths to another location different from where system and Exchange are installed. This way you'll sleep better 😉